1. Field of the Invention
The present invention relates to an improvement in a process for preparing YBa.sub.2 Cu.sub.3 O.sub.7-x superconductors having a high critical current density. More particularly, the present invention is concerned with an isothermal peritectic reaction technique for preparing YBa.sub.2 Cu.sub.3 O.sub.7-x superconducting wires or tapes.
2. Description of the Prior Art
The discovery of high temperature superconductivity in copper-oxide compounds led to intense investigations and researches thereon owing to their immense industrial applicability. In contrast to the initial expectations, the progress toward major applications of the oxide-based superconducting wires and tapes has been hampered by their low critical current density values.
Specifically, polycrystalline YBa.sub.2 Cu.sub.3 O.sub.7-x (hereinafter, referred to as "123") superconductors have a lower critical current density (about 10.sup.2 A/cm.sup.2) compared with single crystal or epitaxially grown thin films. It has been reported that such a low critical current density is mainly due to the anisotropy of electric conductivity, the second phases or impurities present in the grain boundaries, low sintering temperature, the presence of micro-crack and stress zone, and so forth. See, J. W. Ekin, Adv. Ceram. Mat., 2[3B], p.586 (1987); and S. Jin, T. H. Tiefel, R. C. Sherwood, M. E. Davis, R. B. van Dover, G. W. Kammlott, and R. A. Fastnacht, Phys. Rev. B., 37, p.7850 (1988).
It was discovered by Dimos et al. that the ratio of the critical current density at the grain boundaries to the critical current density within the grain decreases rapidly as the anisotropic angle of 123 bicrystal increases. See, P. Chaudhari, J. Mannhart, D. Dimos, C. C. Tsuei, J. Chi, M. M. Oprysko, and M. Scheuermann, Phys. Rev. Lett., 60, p.1653 (1988); D. Dimos, P. Chaudhari, J. Mannhart, and F. K. LeGoues, Phys. Rev. Lett., 61, p.219 (1988); J. Mannhart, P. Chaudhari, D. Dimos, C. C. Tsuei, and T. R. McGuire, Phys. Rev. Lett., 61, p.2476 (1988); and D. R. Clarke, T. M. Shaw, and D. Dimos, J. Am. Ceram. Soc., 72[7], p.1103 (1989). The above discovery indicates that such a low critical current density of oxide-based polycrystalline superconductors depends on the properties intrinsic to the grain boundaries and is affected concomitantly by the extrinsic factors such the second phases, micro-crack, low sintered density, and so on.
In order to improve the critical current density of 123 superconductors, it is necessary to minimize the problems involved in the grain boundaries by aligning the 123 grains in a direction. It has been reported that superconductors having a high critical current density of above 10.sup.4 A/cm.sup.2 can be obtained by aligning the grains in a direction so that only the low angle grain boundaries are arranged in the direction of the current flow. See, S. Jin, T. H. Tiefel, R. C. Sherwood, M. E. Davis, R. B. van Dover, G. W. Kammlott, and R. A. Fastnacht, Phys. Ref. B, 37, p.7850 (1988); M. Murakami, M. Morita, K. Doi, K. Miyamoto, and H. Hamada, Jpn. J. Appl. Phys., 28[3], L399 (1989); M. Murakami, M. Morita, K. Doi, K. Miyamoto, Jpn. J. Appl. Phys, 28[7], p.1189 (1989); M. Murakami, H. Fujimoto, T. Oyama, S. Gotoh, Y. Shiohara, N. Koshizuka, and S. Tanaka, High Tc Information Service, Vol 3, No. 5 (1990); K. Salama, V. Selvamanickam, L. Gao and K. Sun, Appl. Phys. Lett., Vol. 54[23], p.2352 (1989).
The above prior art processes comprises heating a pre-sintered 123 specimen to a temperature higher than the peritectic temperature of 123 to obtain a melt and Y.sub.2 BaCuO.sub.5 (hereinafter, referred to as "211") phase, and very slowly cooling the resultant. The time usually needed to prepare a wire of 1-2 cm long is a few tens of hours. Therefore, the process requires an extremely long period of time and a high temperature of above about 1,000.degree. C.
K. Bose et al. proposes a process for preparing super-conducting wires or tapes, which comprises mixing YBa.sub.2 Cu.sub.3 O.sub.7-x powder with certain polymers, and compressing the resulting mixture into a wire or tape shape followed by sintering below the peritectic temperature of 123. See, K. Bose, "A Simple Method for the Preparation of YBa.sub.2 Cu.sub.3 O.sub.x Superconducting Wires and Tapes Using Common Polymers," Jap. J. Appl. Phys. Vol. 30[5A], p. L823 (1991). According to this process, 123 wires or tapes can be prepared in a continuous manner at a relative low temperature for a short period of time. However, the resulting 123 super-conductors are hardly used in the practical application owing to their critical current density less than about 10.sup.2 A/cm.sup.2.
We, the inventors of the present application, have intensively conducted a wide range of experiments in order to solve the problems encountered in the prior art processes. As a result, the inventors have now developed an isothermal peritectic reaction (IPR) process by which superconductors having a high critical current density can be obtained by heat-treatment at a lower temperature and for a shorter period of time as compared with the conventional melting techniques.